1. Field of Invention
The present invention relates generally to a compounding method for producing impact-modified thermoplastic compositions with a low content of volatile organic compounds (hereinafter VOCs), wherein relatively inexpensive polymer raw materials with a comparatively elevated initial VOC content may be used, resulting in lowered production costs for producing such VOC-reduced polymer compositions.
A compounding method according to the present invention makes this reduction in overall VOC content possible under comparatively mild thermal compounding conditions, which are thereby gentle on the material, i.e. under comparatively low processing temperatures. The decomposition of thermally sensitive components of the compositions is thereby advantageously largely inhibited.
2. Description of Related Art
In polymer processing, compounding describes the production of a finished plastics moulding composition, the compound, from optionally a plurality of polymeric raw materials optionally with the addition of polymer additives such as for example fillers and reinforcing materials, coupling agents, lubricants, stabilisers etc. Compounding predominantly proceeds in kneaders or extruders and comprises the operations of feeding, melting, dispersing, mixing, devolatilising and pressure build-up. Compounding is generally followed by cooling-induced solidification of the compound and pelletisation thereof.
More recently, the requirement has been for the lowest possible content of volatile organic components in plastics mixtures. Devolatilisation during compounding of plastics is accordingly of increased significance. Prior to devolatilisation, the proportion of volatile organic compounds (VOC) in the plastics raw materials (i.e. in the components used for the method) typically amounts to up to 10,000 ppm. It is in practice desired to achieve a VOC content in the finished compound (i.e. for example, in the pellets produced by compounding, cooling and pelletisation) of less than 600 ppm, preferably of less than 400 ppm.
When producing polymer compositions with a low residual volatiles content, said compositions are exposed to a reduced pressure for devolatilisation as a rule in the molten state and at the highest possible temperature, which serves to increase the volatility of the organic substances to be removed from the composition. The reduction in VOCs in the compound brought about thereby is frequently insufficient, however, to meet the increasingly stringent requirements in different fields of application (for example for products for use in motor vehicle interiors or living rooms). This applies in particular to temperature-sensitive polymers or polymer blends, in which the temperature during the devolatilisation process naturally has an upper limit (for example, as explained in detail below, because of the risk of thermally induced elimination of monomers or indeed of thermohydrolytic cleavage of polymer chains with a reduction in the average polymer molecular weight). To achieve the desired low target VOC contents, it is therefore generally necessary, in the prior art methods, only to use polymer raw materials which already have a very low initial VOC content, preferably with a VOC content of at most 600 ppm when producing polymer compositions containing thermosensitive polymers. Such polymer raw materials with low VOC contents have in general to have their VOC content reduced during production thereof by technical measures which are possibly repeated several times, before they are used in the production (compounding) of the polymer composition. Such additional method steps for devolatilising the polymer raw materials have the disadvantage that they result in increased costs relating to production of these polymer raw materials. There is therefore a general need to provide a compounding method which achieves the desired low VOC contents of the target composition even when raw materials with a relatively high VOC content (i.e. for example of more than 600 ppm and up to 2000 ppm) are used, as this would markedly reduce the cost of producing the resultant polymer composition.
In temperature-sensitive polymers or polymer blends, in addition to the residual monomers and solvents to be devolatilised, which are already contained in the raw materials, monomers may also form during the compounding process by thermally induced polymer dissociation. In the case of rubber-modified blends based on a polybutadiene-based impact modifier, such as for example blends of polycarbonate and acrylonitrile-butadiene-styrene terpolymer (PC/ABS blends), this is for example the case with ABS, wherein the polybutadiene contained possibly dissociates to yield 1,3-butadiene. The dissociation kinetics of the polybutadiene is generally highly temperature-dependent, i.e. dissociation increases superproportionally as the temperature increases. The difficulty of achieving process control in the case of compounding lies in the fact that, for effective devolatilisation of the volatile components both a low pressure and a high temperature are necessary, but, at a high temperature, dissociation of the polybutadiene simultaneously greatly increases and thus the 1,3-butadiene content in the final product generally depends less on the concentration thereof in the initial raw materials used and the devolatilisation efficiency of the compounding method and more substantially on the temperature profile during the compounding method. To make matters worse, when energy input is reduced to lower the temperature and avoid dissociation, problems may arise during melting and intermixing of the compound components, which have a negative effect on product quality (i.e. the mechanical properties of the composition). This is true in particular at elevated throughput rates, i.e. short residence times, which are necessary for an economic compounding process.
At the elevated temperatures as would be needed for the necessary devolatilisation according to conventional method according to the previous prior art, and additionally due to the residual moisture content of the polymer raw materials used, polycondensation polymers such as for example polycarbonates, polyesters, polyester carbonates or polyamides may decompose thermohydrolytically with a negative effect on polymer chain length and thus ultimately also on product quality (i.e. the mechanical properties of the composition). To prevent this, complex, cost-intensive predrying of all the raw materials used is generally necessary.
It is in principle also known to use entraining agents to reduce further the residual volatiles content during devolatilisation of polymer melts at a given melt temperature.
WO-A 2008/025446 describes for example a method for compounding polymers and polymer blends, also including PC+ABS compositions, in a screw extruder, wherein the final product has a particularly low content of residual solvents, monomers and oligomers, characterised in that devolatilisation proceeds using an inert entraining agent (for example nitrogen, carbon dioxide or a noble gas), which is introduced into the polymer melt, dispersed therein and subsequently removed again from the product together with the volatile organic compounds by application of reduced pressure in a devolatilising zone. Such a method requires a specific technical design of the extruder with complex extruder peripheral equipment and in this respect cannot generally be implemented using conventional machinery without heavy investment in retrofitting.
In addition, the use of water as an inexpensive entraining agent is known in principle. However, the homogeneous apportioning of water and the homogeneous distribution thereof in the polymer melt necessary for effective devolatilisation is still a technical challenge.
WO-A 2004/069913 describes a method for reducing pollutants and/or odour-emitting substances in polymeric materials using hydrophilic inorganic carrier materials such as zeolites or silicas loaded with 30 to 90 wt. % water as an additive. In this method, however, the hydrophilic inorganic carrier materials remain in the final product and impair the mechanical properties thereof.
EP-A 867 463 and EP-A 1 471 093 describe methods of producing elastomer-modified thermoplastics by mixing elastomer with a thermoplastic, wherein the moist elastomer, in particular a graft polymer, which has been precipitated from a latex, is predewatered to 1 to 50 wt. % residual moisture content and mixed into the thermoplastic present as a melt, characterised in that evaporation of the process water adhering to the elastomer, melting of the elastomer, alloying of the elastomer with the melt of the thermoplastic, and removal of further organic volatile constituents proceeds simultaneously in a process chamber. In this method, the thermoplastic and elastomer are generally used in the range from 1:4 to 4:1. Because of the comparatively long residence time and high thermal loading in this process of the mixture in the process chamber of 2 to 20 minutes and the generally elevated water concentration in the process chamber, this method is not suitable for producing elastomer-modified thermoplastics, in which the thermoplastics are sensitive to hydrolytic degradation.
EP-A 534 235 describes methods of producing impact-modified thermoplastics by compounding rubber latexes which have been mechanically partially dewatered into thermoplastic polymers above the softening points of the thermoplastic polymers, characterised in that the rubbers are partially dewatered in an extruder before being mixed into the thermoplastic polymers and the remaining residual water is removed during compounding by devolatilising devices up- and down-stream of the mixing point. EP-A 534 235 does not disclose any methods for producing rubber-modified polycondensation products, in particular polycarbonates. Due to the comparatively high water concentrations and long residence times at high temperatures generally used in such processes, it may be assumed that they are fundamentally unsuitable for the production of such hydrolysis-sensitive compositions.